In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High levels of condensed moisture inside a cable sheath system may have a detrimental effect on the transmission characteristics of a metallic conductor cable, for example.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, rodent attacks or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter, and, if not controlled, to move longitudinally along the cable into splice closures, for example.
Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment inside closures, for example, may cause problems especially in freezing environments and should be prevented.
Also, optical fiber cables must be provided with sufficient strength to prevent change to the optical fibers when forces are applied to the cables. Generally, this has been accomplished by including a strength member system in a core of an optical fiber cable or in the sheath system or in both.
Many optical fiber cables now include a longitudinally extending tape which has been wrapped about a core of the cables and which includes a superabsorbent material that is capable of swelling upon contact with water to block the flow of water. In U.S. Pat. No. 4,867,526 which issued on Sep. 19, 1989 in the name of C. J. Arroyo, a cable having waterblocking provisions is disclosed. Interposed between a core and a jacket is an elongated substrate member which comprises a non-metallic, non-woven, web-like material in the form of a tape which has been impregnated with a superabsorbent material. The tape material is relatively compressible and has sufficient porosity and superabsorbent material so that it provides enhanced waterblocking capability.
In another prior art cable, a water blockable yarn is interposed between a core and an outer surface of a jacket of the cable's sheath system. The yarn extends linearly along the cable or may be wrapped helically about a portion of the sheath system. The yarn may be one which is comprised of a superabsorbent fiber material which upon contact with water swells and inhibits the movement of water within the cable.
One demanding use for optical fiber cable is aboard ship. Typically in such an environment, the optical fiber cable extends along tortuous paths. Of course, being on a ship, such a cable may be subjected to intense amounts of water until repairs are made, during which time, the cable is expected to remain operational.
In one optical fiber cable, a core comprises at least one transmission media and a plastic jacket and includes provisions for preventing the movement of water within the cable. The cable includes a yarn-like strength member system including longitudinally extending aramid ribrous strength members having a relatively high modulus and having waterblocking provisions. In one embodiment, each fibrous strength member is treated with a superabsorbent liquid material which when dry fills interstices and covers portions of the exterior thereof. In another embodiment, a filamentary strand material comprising a water swellable fibrous material is wrapped about each fibrous strength member.
Although the above-identified cable has been used aboard ships and in other environments in which cable is exposed to water, the strength member system which is treated with a superabsorbent material causes some problems. For example, it has been found to be somewhat difficult to process the treated strength members during manufacture of the cable. Also, it is relatively expensive to coat aramid fibrous yarn with a superabsorbent material. Further, the coating material on the coated yarn tends to be subject to abrasion as the coated yarn is payed off a supply spool to be wrapped about the core.
The consensus is that aramid yarn is a very suitable strength member material for optical fiber cables, but there are shortcomings associated with the impregnation of aramid yarn with a superabsorbent material. It should be apparent that both properties, strength and waterblocking, need to be provided for optical fiber cable, particularly one used on shipboard.
Also, cables for special applications may have more demanding requirements for blocking water than for cable used in commonplace applications. For example, a typical requirement for a cable is that no water flows through a one meter cable sample when a sample of the cable is subjected to a water head, i.e. pressure, of one meter over one hour. In one special application, a cable, to be acceptable, must not allow any more than thirty-three milliliters of sea water to move beyond one meter of cable when subjected to a water head of eleven meters over six hours.
What is sought after and what does not appear to be available is an optical fiber cable which is suitable for shipboard use and which is more easily manufacturable than those of the prior art. The sought after cable must meet stringent requirements for blocking water flow within and into the cable and must have suitable strength to allow the cable to be routed in tortuous paths.